In operation of a high intensity metal halide discharge lamp, visible radiation is emitted by the metal portion of the metal halide fill at relatively high pressure upon excitation typically caused by passage of current therethrough. One class of high intensity, metal halide lamps comprises electrodeless lamps which generate an arc discharge by establishing a solenoidal electric field in the high-pressure gaseous lamp fill comprising the combination of one or more metal halides and an inert buffer gas. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube which contains the fill. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, thus producing a toroidal arc discharge in the arc tube.
High intensity, metal halide discharge lamps, such as the aforementioned electrodeless lamps, generally provide good color rendition and high efficacy in accordance with the principles of general purpose illumination. However, the lifetime of such lamps can be limited by degradation of the arc tube by chemical attack due to ambipolar diffusion. In particular, during lamp operation, the metal halide component of the fill is dissociated and ionized in the arc. By the process of ambipolar diffusion, the positive metal ions are driven towards the arc tube wall by the electric field of the arc column, while the negative halogen ions are retarded from diffusing to the wall. As a result, during initial lamp operation, more metal ions than halogen ions reach the quartz wall of the arc tube. This causes an excess flow of metal atoms (both neutral and ionized) to the quartz wall, resulting in a chemical reaction between the metal and the quartz, which leads to the loss of metal atoms. Disadvantageously, the loss of metal atoms leads to the release of free halogen into the arc tube, which may build up to a level that is high enough to cause arc instability and eventual arc extinction, especially in electrodeless high intensity, metal halide discharge lamps. In addition, the loss of metal atoms shortens the useful life of the lamp by reducing the visible light output.
Accordingly, it is desirable to prevent a substantial loss of the metal component of the metal halide fill and the attendant substantial buildup of free halogen in a high intensity, metal halide discharge lamp.